This document discusses multiple access protocols used at the data link layer. It describes random access protocols like ALOHA and CSMA, controlled access protocols using reservation, polling and token passing, and channelization protocols including FDMA, TDMA and CDMA. Random access allows stations to transmit without coordination when they have data, while controlled access requires stations to get authorization before transmitting. Channelization divides the available bandwidth using techniques like frequency, time, or code to allow multiple simultaneous transmissions. Diagrams and examples are provided to illustrate how each protocol works.
4. 12.4
12-1 RANDOM ACCESS12-1 RANDOM ACCESS
InIn random accessrandom access oror contentioncontention methods, no station ismethods, no station is
superior to another station and none is assigned thesuperior to another station and none is assigned the
control over another. No station permits, or does notcontrol over another. No station permits, or does not
permit, another station to send. At each instance, apermit, another station to send. At each instance, a
station that has data to send uses a procedure definedstation that has data to send uses a procedure defined
by the protocol to make a decision on whether or notby the protocol to make a decision on whether or not
to send.to send.
ALOHA
Carrier Sense Multiple Access
Carrier Sense Multiple Access with Collision Detection
Carrier Sense Multiple Access with Collision Avoidance
Topics discussed in this section:Topics discussed in this section:
7. 12.7
A pure ALOHA network transmits 200-bit frames on a
shared channel of 200 kbps. What is the requirement to
make this frame collision-free?
Example 12.2
Solution
Average frame transmission time Tfr is 200 bits/200 kbps
or 1 ms. The vulnerable time is 2 × 1 ms = 2 ms. This
means no station should send later than 1 ms before this
station starts transmission and no station should start
sending during the one 1-ms period that this station is
sending.
16. 12.16
A network using CSMA/CD has a bandwidth of 10 Mbps.
If the maximum propagation time (including the delays in
the devices and ignoring the time needed to send a
jamming signal, as we see later) is 25.6 μs, what is the
minimum size of the frame?
Example 12.5
Solution
The frame transmission time is Tfr = 2 × Tp = 51.2 μs.
This means, in the worst case, a station needs to transmit
for a period of 51.2 μs to detect the collision. The
minimum size of the frame is 10 Mbps × 51.2 μs = 512
bits or 64 bytes. This is actually the minimum size of the
frame for Standard Ethernet.
20. 12.20
In CSMA/CA, the IFS can also be used to
define the priority of a station or a
frame.
Note
21. 12.21
In CSMA/CA, if the station finds the
channel busy, it does not restart the
timer of the contention window;
it stops the timer and restarts it when
the channel becomes idle.
Note
23. 12.23
12-2 CONTROLLED ACCESS12-2 CONTROLLED ACCESS
InIn controlled accesscontrolled access, the stations consult one another, the stations consult one another
to find which station has the right to send. A stationto find which station has the right to send. A station
cannot send unless it has been authorized by othercannot send unless it has been authorized by other
stations. We discuss three popular controlled-accessstations. We discuss three popular controlled-access
methods.methods.
Reservation
Polling
Token Passing
Topics discussed in this section:Topics discussed in this section:
27. 12.27
12-3 CHANNELIZATION12-3 CHANNELIZATION
ChannelizationChannelization is a multiple-access method in whichis a multiple-access method in which
the available bandwidth of a link is shared in time,the available bandwidth of a link is shared in time,
frequency, or through code, between different stations.frequency, or through code, between different stations.
In this section, we discuss three channelizationIn this section, we discuss three channelization
protocols.protocols.
Frequency-Division Multiple Access (FDMA)
Time-Division Multiple Access (TDMA)
Code-Division Multiple Access (CDMA)
Topics discussed in this section:Topics discussed in this section: